The use of perfluorinated ion-exchange membranes is rapidly expanding as the preferred energy-efficient technology for the electrolysis of brine to produce caustic and chlorine. Typical electrolytic cells used for this purpose comprise an anode and a cathode, an anode compartment and a cathode compartment, and the perfluorinated ion-exchange membrane situated so as to separate the two compartments. Brine is fed into the anode compartment, and a current is caused to flow through the cell.
It has been found that certain impurities in the brine feed can adversely affect the electrolysis process by reducing the performance and useful life of the ion-exchange membrane. One such common impurity in brine is silica (SiO.sub.2). In the cell, silica can move through the membrane and precipitate as a complex with aluminum or calcium in the membrane layer adjacent to the catholyte. (Research Disclosure, July 1984, page 348, Item 24337, "Effect of Aluminum and Silica Impurities in Brine on Membrane Performance in Chloralkali Cells".)
To avoid membrane damage caused by silica, aluminum and calcium, prior practice has been to limit the concentrations of these cations in the brine feed to fixed levels. (This practice was not, however, apparently based on any recognition that silica could form membrane-damaging precipitates with calcium or aluminum). For example, U.S. Pat. No. 4,450,057, issued May 22, 1984, discloses a process for removing dissolved aluminum and silica contaminants from alkali metal halide brines involving contacting an acidified brine at a pH of between 2.0 and 3.0 with a strong macroreticular cationic chelating resin. U.S. Pat. No. 4,155,820, issued May 22, 1979, discloses a process for removing silica from aqueous sodium chloride solution by coprecipitation. The patent states that the amount of soluble silica in feed brine should be reduced to 4 ppm or less to avoid increases in electrolysis voltage. These practices, however, are not entirely satisfactory because they do not prevent membrane damage in all circumstances and often cause the cell operator added expense.